Cation-like Doppler shifts from a neutral molecule in an electrical discharge

Velocity-modulation Fourier transform emission spectra from a N_2O/He discharge plasma recorded between 1 and 5.5 $\mu$m are described. Surprisingly, they show Doppler-shifted lines for the E ^2 \Sigma ^+ - D ^2 \Sigma ^+, C ^2 \Pi - A ^2 \Sigma ^+, and D ^2 \Sigma ^+ - A ^2 \Sigma ^+ Rydberg-Rydberg rovibronic transitions of the nitric oxide neutral molecule. These polarity-dependent Doppler-shifts are those of positively charged particles. Vibration-rotation lines of NO and transitions from other neutral molecules like N_2 are also present in the spectra with comparable intensities and remain unshifted. Experimental investigations and possible explanations are discussed.